The present invention relates to a door closer, especially to a hydraulically controlled door closer with a closer shaft biased by a spring arrangement in door closing position, in which the closer shaft is connected to a piston reciprocable in fluid filled cylinder whereby between the pressure space and the pressureless space in the cylinder a further piston is arranged provided with a one-way valve opening toward the pressure space and an electromagnetically operated valve preventing in the closed position flow of fluid between the two spaces.
The U.S. Pat. No. 3,852,846 discloses a door closer in which two pistons are arranged in a fluid filled cylinder, of which one of the pistons is an operating piston provided with a rack cooperating with a pinion fixed to the closer shaft, whereas the other piston is constructed as a holding piston abutting against the operating piston for holding the latter, by forming a pressure space between the supporting piston and a plug closing one end of the cylinder and provided with an electromagnetically operated valve, in any position when the aforementioned pressure space is closed by the electromagnetically operated valve. In this construction the operating piston is continuously biased in closing direction by a spring arrangement which holds the operating piston in continuous abutment with the supporting piston. The armature of the electromagnetic valve is connected to a valve member which engages in adjacent connecting passages formed in the aforementioned plug and which, depending on the position of the armature, opens, respectively closes, the connecting passages between the pressure space and the pressureless space in which the operating piston is arranged. To prevent pressure build-up in the space in which the operating piston is arranged equalizing channels are provided in the wall of the cylinder in the region of movement of the operating piston, in which throttle valves for damping the closure movement of the door are provided. When the electromagnet is deenergized, the connecting channels between the pressure space behind the supporting piston and the pressureless space in front of the supporting piston are connected to each other and the pressure medium is pressed, during closing of the door by the spring arrangement biasing the operating piston, over the supporting piston abutting against the operating piston from the pressure space into the pressureless space. If the electromagnet is energized, its armature is attracted to close with the valve member connected thereto the connecting conduit so that the supporting piston will hold a door in an open position reached by the door at the moment the electromagnet is energized. The desired opening angle at which the door is to be held in open position may, therefore, be freely selected. A further opening of the door beyond the arrested open position up to its maximal open position is, however, possible against the force of the spring arrangement biasing the operating piston since the equalizing channels in the cylinder wall in the region of movement of the operating piston permit a movement of the latter in opening direction. During opening of the door provided with the above-described door closer it is, therefore, necessary to overcome the force of the spring, tending to move the door to the closed position, and the door will also be moved back from its maximum open position to the predetermined open position when the door is released. The door closing function of the spring can, therefore, not be nullified with this known door closer.